Process and catalyst for producing syndiotactic polyolefins

ABSTRACT

The invention provides a metallocene catalyst for use in preparing syndiotactic polyolefins. The catalyst comprises a bridged metallocene in which one of the cyclopentadienyl rings is substituted in a substantially different manner from the other ring. It was discovered that this type of catalyst is highly syndiospecific, and it also produces a polymer with a novel microstructure. The invention further includes the use of one or more of the catalysts in a polymerization process. The catalyst is generally described by the formula 
     
         R&#34;(CpR.sub.n)(CpR&#39;.sub.m)MeQ.sub.k 
    
     wherein each Cp is a cyclopentadienyl or substituted cyclopentadienyl ring; each R n  and R&#39; m  is the same or different and is a hydrocarbyl radicals having 1-20 carbon atoms; R&#34; is a structural bridge between the two Cp rings imparting stereoigidity to the catalyst; Me is a group 4b, 5b, or 6b metal from the Periodic Table of Elements; each Q is a hydrocarbyl radical having 1-20 carbon atoms or is a halogen; 0≦k≦3; 0≦n≦4; 1≦m≦4; and wherein R&#39; m  is selected such that (CpR&#39; m ) is a sterically different ring than (CpR n ).

TECHNICAL FIELD

The invention relates to a metallocene catalyst useful in preparingsyndiotactic polyolefins. The catalyst consists of a bridged metallocenein which one of the cyclopentadienyl rings is substituted in a differentmanner from the other ring. The invention further includes a process ofpreparing syndiotactic polyolefins that comprises the use of one or moreof the disclosed catalysts and also a process for preparing thecatalysts.

BACKGROUND OF THE INVENTION

The present invention provides a catalyst and process for polymerizingolefins having three or more carbon atoms to produce a polymer with asyndiotactic stereochemical configuration. The catalyst and process areparticularly useful in polymerizing propylene to form a highlycrystalline, novel microstructure of syndiotactic polypropylene.

As known in the art, syndiotactic polymers have a unique stereochemicalstructure in which monomeric units having enantiomorphic configurationof the asymmetrical carbon atoms follow each other alternately andregularly in the macromolecular main chain. Syndiotactic polypropylenewas first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. TheNatta group obtained syndiotactic polypropylene by using a catalystprepared from titanium trichloride and diethyl aluminum monochloride. Alater patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the useof vanadium triacetylacetonate or haloenated vanadium compounds incombination with organic aluminum compounds for producing syndiotacticpolypropylene. U.S. Pat. No. 3,364,190 to Emrick discloses a catalystsystem composed of finely divided titanium or vanadium trichloride,aluminum chloride, a trialkyl aluminum and a phosphorus-containing Lewisbase as producing syndiotactic polypropylene.

As disclosed in these patent references and as known in the art, thestructure and properties of syndiotactic polypropylene differsignificantly from those of isotactic polypropylene. The isotacticstructure is typically described as having the methyl groups attached tothe tertiary carbon atoms of successive monomeric units on the same sideof a hypothetical plane through the main chain of the polymer, e.g., themethyl groups are all above or below the plane. Using the Fischerprojection formula, the stereochemical sequence of isotacticpolypropylene is described as follows: ##STR1##

Another way of describing the structure is through the use of NMR.Bovey's NMR nomenclature for an isotactic pentad is ...mmmm... with each"m" representing a "meso" dyad or successive methyl groups on the sameside in the plane. As known in the art, any deviation or inversion inthe structure of the chain lowers the degree of isotacticity andcrystallinity of the polymer.

In contrast to the isotactic structure, syndiotactic polymers are thosein which the methyl groups attached to the tertiary carbon atoms ofsuccessive monomeric units in the chain lie on alternate sides of theplane of the polymer. Syndiotactic polypropylene is shown in zig-zagrepresentation as follows: ##STR2## Using the Fischer projectionformula, the structure of a syndiotactic polymer is designated as:##STR3## In NMR nomenclature, this pentad is described as ... rrrr... inwhich each "r" represents a "racemic" dyad, i.e., successive methylgroups on alternate sides of the plane. The percentage of r dyads in thechain determines the degree of syndiotacticity of the polymer.Syndiotactic polymers are crystalline and, like the isotactic polymers,are insoluble in xylene. This crystallinity distinguishes bothsyndiotactic and isotactic polymers from an atactic polymer that issoluble in xylene. Atactic polymer exhibits no regular order ofrepeating unit configurations in the polymer chain and forms essentiallya waxy product.

While it is possible for a catalyst to produce all three types ofpolymer, it is desirable for a catalyst to produce predominantlyisotactic or syndiotactic polymer with very little atactic polymer.Catalysts that produce isotactic polyolefins are disclosed in copendingU.S. Patent Application Ser. Nos. 034,472 filed Apr. 3, 1987 and nowabandoned; 096,075 filed Sept. 11, 1987 and now Pat. No. 4,794,096; and095,755 filed on Sept. 11, 1987 and now abandoned. These applicationsdisclose chiral, stereorigid metallocene catalysts that polymerizeolefins to form isotactic polymers and are especially useful in thepolymerization of a highly isotactic polypropylene. The presentinvention, however, provides a different class of metallocene catalyststhat are useful in the polymerization of syndiotactic polyolefins, andmore particularly syndiotactic polypropylene.

In addition to a newly discovered catalyst, the present invention alsoprovides syndiotactic polypropylene with a new microstructure. It wasdiscovered that the catalyst structure not only affected the formationof a syndiotactic polymer as opposed to an isotactic polymer, but italso appears to affect the type and number of deviations in the chainfrom the principally repeating units in the polymer. Previously, thecatalysts used to produce syndiotactic polypropylene were believed toexercise chain-end control over the polymerization mechanism. Thesepreviously known catalysts, such as the ones disclosed by Natta et al inthe references cited above, produce predominately syndiotactic polymershaving the structure ##STR4## or in NMR nomenclature ...rrrrrmrrrrr....The NMR analysis for this structure of syndiotactic polypropylene isshown in Zambelli, et al., Macromolecules, Vol. 13, pp 267-270 (1980).Zambelli's analysis shows the predominance of the single meso dyad overany other deviation in the chain. It was discovered, however, that thecatalysts of the present invention produce a polymer with a differentmicrostructure than that previously known and disclosed, and in additionone having a high percentage of racemic dyads in the structure.

SUMMARY OF THE INVENTION

The present invention provides a catalyst and process for preparingsyndiotactic polyolefins, and more particulary syndiotacticpolypropylene. The catalyst and process produce a polymer with a highsyndiotactic index and with a novel syndiotactic microstructure.Further, the invention includes a process for producing syndiotacticpolypropylene having a broad molecular weight distribution and a processfor tailoring the characteristics of the polymer such as melting pointby varying the structure of the catalyst.

The novel catalyst provided by the present invention is a stereorigidmetallocene catalyst described by the formula:

    R"(CpR.sub.n)(CpR'.sub.m)MeQ.sub.k

wherein each Cp is a cyclopentadienyl or substituted cyclopentadienylring; each R_(n) and R'_(m) is a hydrocarbyl radical having 1-20 carbonatoms; R" is a structural bridge between the two Cp rings impartingstereorigidity to the Cp rings; Me is a transition metal; and each Q isa hydrocarbyl radical or is a halogen. Further, R'_(m) is selected sothat (CpR'_(m)) is a sterically different substituted cyclopentadienylring than (CpR_(n)). It was discovered that the use of a metallocenecatalyst with sterically different cyclopentadienyl rings produces apredominantly syndiotactic polymer rather than an isotactic polymer.

The present invention further provides a process for producingsyndiotactic polyolefins, and particularly, syndiotactic polypropylene,that comprises utilizing at least one of the catalysts described by theabove formula and introducing the catalyst into a polymerizationreaction zone containing an olefin monomer. In addition, an electrondonor compound and/or a cocatalyst such as alumoxane may be introducedinto the reaction zone. Further, the catalyst may also bepre-polymerized prior to introducing it into the reaction zone and/orprior to the stabilization of reaction conditions in the reactor.

The present invention also includes a process for producing syndiotacticpolyolefins having a broad molecular weight distribution. This processcomprises utilizing at least too different catalysts described by theabove formula in the process of polymerization.

It was further discovered that the characteristics of the polymerproduced by the process of polymerization described herein could becontrolled by varying the polymerization temperature or the structure ofthe catalyst. In particular, it was discovered that a higherpolymerization temperature resulted in a syndiotactic polymer with amixed microstructure. Also, it was discovered that the melting points ofthe polymer are affected by the reaction temperature, thecatalyst-cocatalyst ratio, and the structure of the catalyst. A higherreaction temperature generally produces a less crystalline polymerhaving a lower melting point. Further, polymer products having differentmelting points are obtainable by varying the structure of the catalyst.

The present invention further includes a process for preparing a bridgedmetallocene catalyst comprising contacting a cyclopentadiene orsubstituted cyclopentadiene with fulvene or a substituted fulvene underreaction conditions sufficient to produce a bridged dicyclopentadiene orsubstituted dicyclopentadiene. The process further comprises contactingthe bridged dicyclopentadiene with a metal compound of the formulaMeQ_(k) as defined above under reaction conditions sufficient to complexthe bridged dicyclopentadiene to produce a bridge metallocene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the structure of a preferred catalyst ofthe present invention and specifically showsiso-propyl(cyclopentadienyl) (fluorenyl)hafnium dichloride.

FIG. 2 is an NMR spectra for the polymer produced in Example 1 usingiso-propyl(cyclopentadienyl) (fluorenyl) zirconium dichloride. Thepolymer was recrystallized once from xylene.

FIGS. 3 and 4 are IR spectra for the polymers produced in Examples 7 and8 respectively with the polymer being recrystallized three times fromxylene.

DETAILED DESCRIPTION

The present invention provides a catalyst and process for the productionof syndiotactic polyolefins, particularly polypropylene. Not only do thecatalysts of the present invention produce syndiotactic polypropylene,but they also produce a polymer with a novel microstructure.

When propylene or other alpha-olefins are polymerized using a catalystconsisting of a transition metal compound, the polymer product typicallycomprises a mixture of amorphous atactic and crystalline xyleneinsoluble fractions. The crystalline fraction may contain eitherisotactic or syndiotactic polymer, or a mixture of both. Highlyiso-specific metallocene catalysts are disclosed in copending U.S.Application Ser. Nos. 034,472; 096,075; and 095,755. In contrast to thecatalysts disclosed in those applications, the catalysts of the presentinvention are syndio-specific and produce a polymer with a highsyndiotactic index. It was discovered that syndiotactic polymersgenerally have lower heats of crystallization than the correspondingisotactic polymers. In addition, for the same number of imperfections inthe polymer chain, syndiotactic polymers have a higher melting pointthat isotactic polymers.

The metallocene catalysts of the present invention may be described bythe formula R"(CpR_(n))(CpR'_(m)) MeQ_(k) wherein each Cp is acyclopentadienyl or substituted cyclopentadienyl ring; R_(n) and R'_(m)are hydrocarbyl radicals having 1-20 carbon atoms, each R_(n) may be thesame or different, and each R'_(m) also may be the same or different; R"is a structural bridge between the two Cp rings imparting stereorigidityto the Cp rings within the catalyst, and R" is preferably selected fromthe group consisting of an alkyl radical having 1-4 carbon atoms or ahydrocarbyl radical containing silicon, germanium, phosphorus, nitrogen,boron, or aluminum; Me is a group 4b, 5b, or 6b metal from the PeriodicTable of Elements; each Q is a hydrocarbyl radical having 1-20 carbonatoms or is a halogen; 0 ≦ k ≦ 3; 0 ≦ n ≦ 4; and 1 ≦ m ≦ 4. In order tobe syndio-specific, it was discovered that the Cp rings in themetallocene catalysts must be substituted in a substantially differentmanner so that there is a steric difference between the two Cp rings,and therefore, R'_(m) is selected such that (CpR'_(m)) is asubstantially different substituted ring than (CpR_(n)). In order toproduce a syndiotactic polymer, the characteristics of the groupssubstituted directly on the cyclopentadienyl rings seem to be important.Thus, by "steric difference" or "sterically different" as used herein,it is intended to imply a difference between the steric characteristicsof the Cp rings that controls the approach of each successive monomerunit that is added to the polymer chain. The steric difference betweenthe Cp rings acts to block the approaching monomer from a randomapproach and controls the approach such that the monomer is added to thepolymer chain in the syndiotactic configuration.

Without intending to limit the scope of the present invention asindicated by the claims, it is believed that in the polymerizationreaction both the catalyst and the approaching monomer units isomerizewith each monomer addition to the polymer chain. This isomerization ofthe monomer which is controlled by the steric blockage of thedifferently substituted Cp rings results in the alternatingconfiguration characteristic of syndiotactic polymers and is in contrastto the chain-end control of the catalysts disclosed by Natta et al. Thedifferent reaction mechanism also results in a different structure forthe polymer.

In a preferred catalyst of the present invention, Me is titanium,zirconium or hafnium; Q is preferably a halogen, and it is mostpreferably chlorine; and k is preferably 2, but it may vary with thevalence of the metal atom. Exemplary hydrocarbyl radicals includemethyl, ethyl, propyl, isopropyl, butyl, isobutyl, amyl, isoamyl, hexyl,heptyl, octyl, nonyl, decyl, cetyl, phenyl, and the like. Otherhydrocarbyl radicals useful in the present catalysts include otheralkyl, aryl, alkenyl, alkylaryl or arylalkyl radicals. Further, R_(n)and R'_(m) may comprise hydrocarbyl radicals attached to a single carbonatom in the Cp ring as well as radicals that are bonded to two carbonatoms in the ring. FIG. 1 shows the structure of a preferred catalystisopropyl(fluorenyl) (cyclopentadienyl) hafnium dichloride. Thezirconium analogue of the catalyst shown in FIG. 1 is similarlypreferred.

The catalyst may be prepared by any method known in the art. TheExamples below disclose two methods of preparing the catalyst with thesecond method being preferred as it produces a more stable and activecatalyst. It is important that the catalyst complex be "clean" asusually low molecular weight, amorphous polymer is produced by impurecatalysts. Generally, the preparation of the catalyst complex consistsof forming and isolating the Cp or substituted Cp ligands which are thenreacted with a halogenated metal to form the complex.

The metallocene catalysts of the present invention are useful in many ofthe polymerization processes known in the art including many of thosedisclosed for the preparation of isotactic polypropylene. When thecatalysts of the present invention are used in these types of processes,the processes produce syndiotactic polymers rather than isotacticpolymers. Further examples of polymerization processes useful in thepractice of the present invention include those disclosed in U.S.Application Ser. No. 009,712 filed on Feb. 2, 1987 and now U.S. Pat. No.4,767,735 and U.S. Application Ser. No 095,755 filed on Sept. 11, 1987and now abandoned, the disclosures of which are hereby incorporatedherein by reference. These preferred polymerization procedures includethe step of prepolymerizing the catalyst and/or precontacting thecatalyst with a cocatalyst and an olefin monomer prior to introducingthe catalyst into a reaction zone.

Consistent with the prior disclosures of metallocene catalysts for theproduction of isotactic polymers, the syndio-specific catalysts of thepresent invention are particularly useful in combination with analuminum cocatalyst, preferably an alumoxane, an alkyl aluminum, or amixture thereof. In addition, a complex may be isolated between ametallocene catalyst as described herein and an aluminum cocatalyst inaccordance with the teachings of European Patent Publication No. 226,463published on June 24, 1987 and assigned to Exxon Chemical Patents Inc.with Howard Turner listed as the inventor. As disclosed therein, ametallocene is reacted with an excess of alumoxane in the presence of asuitable solvent. A complex of the metallocene and alumoxane may beisolated and used as a catalyst in the present invention.

The alumoxanes useful in combination with the catalysts of the presentinvention, either in the polymerization reaction or in forming thecomplex disclosed in Turner, may be represented by the general formula(R--Al--0--) in the cyclic form and R(R--Al--0)_(n) --ALR2 in the linearform wherein R is an alkyl group with one to five carbon atoms and n isan integer from 1 to about 20. Most preferably, R is a methyl group. Thealumoxanes can be prepared by various methods known in the art.Preferably, they are prepared by contacting water with a solution oftrialkyl aluminum, such as, trimethyl aluminum, in a suitable solventsuch as benzene. Another preferred method includes the preparation ofalumoxane in the presence of a hydrated copper sulfate as described inU.S. Pat. No. 4,404,344 the disclosure of which is hereby incorporatedby reference. This method comprises treating a dilute solution oftrimethyl aluminum in toluene with copper sulfate. The preparation ofother aluminum cocatalysts useful in the present invention may beprepared by methods known to those skilled in the art.

The Examples given below illustrate the present invention and itsvarious advantages and benefits in more detail. Two different synthesisprocedures, designated as A and B, are described for both zirconium andhafnium metallocene catalysts. The synthesis procedures in both methodswere performed under an inert gas atmosphere using a Vacuum Atmospheresglovebox or Schlenk techniques. The synthesis process generallycomprises the steps of (1) preparing the halogenated or alkylated metalcompound, (2) preparing the ligand, (3) synthesizing the complex, and(4) purifying the complex. The synthesis of the bridged, substituteddicyclopentadienyl ligand was accomplished by contacting fulvene or asubstituted fulvene with a cyclopentadienyl or substitutedcyclopentadienyl under reaction conditions sufficient to produce abridged dicyclopentadiene or substituted dicyclopentadiene. As known inthe art, fulvene is Cp═C in which a carbon atom is bound by a doublebond to a cyclopentadienyl ring. Substituted fulvene as used herein isintended to mean (CpR_(a))═CR'_(b) wherein fulvene is substituted eitheron the Cp ring or at the terminal carbon atom or both. R_(a) and R_(b) 'are hydrocarbyl radicals, with each R_(a) and R_(b) ' being the same ordifferent, and 0 ≦ a ≦ 4 and 0 ≦ b ≦ 2. The other three steps of thesynthesis may be performed as shown below or other methods known in theart. The general catalyst formula for the catalyst produced by thesemethods is iso-propyl(fluorenyl) (cyclopentadienyl) MeC1₂ wherein Me iseither zirconium or hafnium depending on the example. FIG. 1 shows thestructure of the hafnium catalyst, and the zirconium catalyst hasessentially the same structure with Zr positioned in place of the Hfatom.

Preparation of the Catalyst Method A

In Method A, the halogenated metal compound was prepared usingtetrahydrofuran ("THF") as a solvent resulting in THF bound in with thefinal catalyst complex. Specifically, MeC1₄ THF was prepared asdescribed in Manzer, L., Inorg. Synth., 21, 135-36 (1982). In theExamples below, Me is zirconium and hafnium, but it may also includetitanium or other transition metals.

The substituted dicyclopentadienyl ligand may be prepared using variousprocesses known in the art depending upon the selection of the specificbridge or ring substituents. In the preferred embodiments shown in theExamples below, the ligand is 2,2-isopropyl(fluorene)cyclopentadiene. Toprepare this ligand, 44 gms (0.25 mol) of fluorene were dissolved in 350ml THF in a round bottom flask equipped with a side arm and droppingfunnel. Contained within the funnel were 0.25 mol of methyl lithium (CH₃Li) in ether (1.4 M). The CH₃ Li was added dropwise to the fluorenesolution and the deep orange-red solution was stirred for several hours.After gas evolution had ceased, the solution was cooled to -78° C. and100 ml of THF containing 26.5 gms (0.25 mol) of 6,6-dimethylfulvene wasadded dropwise to the solution. The red solution was gradually warmed toroom temperature and stirred overnight. The solution was treated with200 ml of water and stirred for ten minutes. The organic fraction of thesolution was extracted several times with 100 ml portions ofdiethylether, and the combined organic phases were dried over magnesiumsulfate. Removal of the ether from the organic phases left a yellowsolid which was dissolved in 500 ml of chloroform and recrystallized byaddition of excess methanol at 2° C. to yield a white powder.

The elemental analysis of the ligand showed carbon to be 91.8% by weightof the compound and hydrogen to be 7.4% by weight. This corresponds tothe weight percentages for C₂₁ H₂₀, 92.6% carbon and 7.4% hydrogen. TheNMR spectrum for the ligand establishes the structure to include onecyclopentadienyl ring attached by an isopropyl bridge to a secondcyclopentadienyl ring that is substituted to form a fluorenyl radical.

A syndio-specific catalyst complex was synthesized using the ligand andthe metal tetrachloride-THF complex. The catalyst was formed by adding0.05 mol of N-butyl lithium hexane (1.6M) dropwise to a 100 ml THFsolution containing 6.8 gms (0.025 mol) of the Cp ligand describedabove. The solution was stirred at 35° C. for twelve hours after which9.4 gms (0.025 mol) of ZrC1₄ -2THF contained in 200 ml of THF wererapidly cannulated together with the ligand solution into a 500 ml roundbottom flask with vigorous stirring. The deep orange-red solution wasstirred for twelve hours under reflux. A mixture of LiC1 red solid wereisolated by removing the solvents under vacuum.

Catalyst complexes produced in accordance with Method A are noted to besomewhat impure and extremely air and moisture sensitive. As a result,in the Examples below, Method A catalysts were purified using one ormore of the following purification procedures:

1. Extraction with pentane. Trace quantities of a yellow impuritycontained in the solid red catalyst complex were repeatedly extractedwith pentane until the pentane became colorless.

2. Fractional recrystallization. The red complex was separated from thewhite LiC1 by dissolving it in 100 ml of toluene, filtering it through afine porosity sintered glass frit, and forming a saturated solution byadding pentane. The red zirconium complex was isolated usingcrystallization at -20° C.

3. Chromotography on bio-beads. 50 gms of bio-beads SM-2 (20-50 meshspherical, macroreticular styrene-divinylbenzene copolymer from Bio-Radlaboratories) were dried under vacuum at 70° C. for 48 hours in a 30 ×1.5 centimeter column. The beads were then equilibrated with toluene forseveral hours. A concentrated solution of the red catalyst complex intoluene was eluted down the column with 150-200 ml of toluene. Thecomplex was recovered by evaporating the toluene under vacuum.

Catalyst Synthesis Procedure Method B

As an alternative synthesis procedure, Method B provides catalysts thatare more air stable, more active, and produce a higher percentage ofsyndiotactic polypropylene. In this process, methylene chloride is usedas a non-coordinating solvent. The process described below uses hafniumas the transition metal, but the procedure is adaptable for use withzirconium, titanium or other transition metals. The substituteddicyclopentadienyl ligand was synthesized in THF in the same manner asdescribed in Method A above. The red dilithio salt of the ligand (0.025mol) was isolated as disclosed in Method A by removing the solventsunder vacuum and by washing with pentane. The isolated red dilithio saltwas dissolved in 125 ml of cold methylene chloride and an equivalentamount (0.025 mol) of HfC1₄ was separately slurried in 125 ml ofmethylene chloride at -78° C. The HfC1₄ slurry was rapidly cannulatedinto the flask containing the ligand solution. The mixture was stirredfor two hours at -78° C., allowed to warm slowly to 25° C. and stirredfor an additional 12 hours. An insoluble white salt (LiC1) was filteredoff. A moderately air sensitive, yellow powder was obtained by coolingthe brown/yellow methylene chloride solution to -20° C. for 12 hours andcannulating away the supernatant. The bright yellow product was washedon the sintered glass filter by repeatedly filtering off coldsupernatant that had been cannulated back over it. The catalyst complexwas isolated by pumping off the solvents using a vacuum, and it wasstored under dry, deoxygenated argon. The process yielded 5.5 gms ofcatalyst complex.

The elemental analysis of the hafnium catalyst complex prepared usingMethod B showed that the catalyst consisted of 48.79% by weight ofcarbon, 3.4% hydrogen, 15.14% chlorine and 33.2% hafnium. Thesepercentages compare with the theoretical analysis for C₂₁ H₁₈ HfC1₂which is 48.39% carbon, 3.45% hydrogen, 13.59% chlorine and 34.11%hafnium. Similarly, zirconium catalysts produced using Method B showelemental analysis close to the expected or theoretical values. Further,some of the hafnium complexes illustrated in the Examples below weremade using 96% pure HfC1₄ which also contains about 4% ZrC1₄. Stillother catalyst samples were made using 99.99% pure HfC1₄. Differencescan be seen in the molecular weight distributions of the polymersproduced with the pure Hf catalyst compared with the polymers producedusing the catalysts which contain a small percentage of zirconium. Themixed catalyst produces a polymer with a broader molecular weightdistribution than that produced by a pure catalyst system.

The Examples below illustrate the present invention and its variousadvantages in more detail. The results of the polymerization process andthe analysis of the polymer are shown in Table 1 for Examples 1-17 andTable 2 for Examples 18-33.

EXAMPLE 1

The polymerization of propylene was carried out using 0.16 mg ofisopropyl(cyclopentadienyl)(florenyl) zirconium dichloride produced inaccordance with Method A described above. The catalyst was purifiedusing fractional recrystallization. The catalyst was precontacted for 20minutes with a toluene solution containing 10.7% by weight ofmethylalumoxane (MAO) with an average molecular weight of about 1300.The alumoxane serves as a co-catalyst in the polymerization reaction.Ten cc of the MAO solution was used in the polymerization. The catalystand co-catalyst solution was then added to a Zipperclave reactor at roomtemperature followed by the addition of 1.2 liters of liquid propylene.The reactor contents were then heated to the polymerization temperature,T as shown in Tables 1 and 2, of 20° C. in less than about 5 minutes.During this time, prepolymerization of the catalyst occurred. Thepolymerization reaction was allowed to run for 60 minutes during whichtime the reactor was maintained at the polymerization temperature. Thepolymerization was terminated by rapidly venting the monomer. Thereactor contents were washed with 50% methanol in dilute HC1 solutionand dried in vacuo. The polymerization yielded 14 gms of polypropylene"as polymerized", i.e., without any further isolations or purification.

Analysis of Polymer

The polymer was analyzed to determine the melting point Tm, the heat ofcrystallization Hc, the molecular weights Mp, Mw, and M_(n), the percentof xylene insolubles XI, and the syndiotactic index S.I. Unlessotherwise noted, the analyses were performed on the xylene insolublefraction of the polymer which includes the syndiotactic fraction and anyisotactic polymer produced. The atactic polymer was removed bydissolving the polymer product in hot xylene, cooling the solution to 0°C. and precipitating out the xylene insoluble fraction. Successiverecrystallizations performed in this manner result in removingessentially all atactic polymer from the xylene insoluble fraction.

The melting points, Tm, were derived using Differential ScanningCalorimetry (DSC) data as known in the art. The melting points, Tm1 andTm2 listed in Tables 1 and 2 are not true equilibrium melting points butare DCS peak temperatures. In polypropylene, it is not unusual to get anupper and a lower peak temperature, i.e., two peaks, and both meltingpoints are reported in Tables 1 and 2 with the lower melting pointreported as Tm1 and the higher point as Tm2. True equilibrium meltingpoints obtained over a period of several hours would most likely beseveral degrees higher than the DSC lower peak melting points. As isknown in the art, the melting points for polypropylene are determined bythe crystallinity of the xylene insoluble fraction of the polymer. Thishas been shown to be true by running the DSC melting points before andafter removal of the xylene soluble or atactic form of the polymer. Theresults showed only a difference of 1-2° C. in the melting points aftermost of the atactic polymer was removed. As shown in Table 1, themelting points Were determined to be 145° C. and 150° C. for the polymerproduced in Example 1. DSC data was also used to determine the heat ofcrystallization, -Hc as shown in Tables 1 and 2, measured in joules pergram J/g. The melting points and -Hc were determined on the "aspolymerized" sample before the atactic polymer was removed.

The molecular weights of the polymer were calculated using GelPermeation Chromotography (GPC) analysis done on a Waters 150° C.instrument with a column of Jordi gel and an ultra-high molecular weightmixed bed. The solvent was trichlorobenzene and the operatingtemperature was 140° C. From GPC, M_(p) which is the peak molecularweight, M_(n) which is the number average molecular weight and M_(w)which is the weight average molecular weight were derived for the xyleneinsolube fraction of the polymer produced. The molecular weightdistribution, MWD, is commonly measured as M_(w) divided by M_(n). Thevalues determined for this sample are shown in Table 1. GPC analysis wasalso used to determine the syndiotactic index, S.I.%, shown in Tables 1and 2. The syndiotactic index is a measure of the percentage of thesyndiotactic structure produced in the polymerization reaction and wasdetermined from the molecular weight data on the samples "aspolymerized."

NMR analysis was used to determine the microstructure of the polymer. Asample of the polymer produced above was dissolved in a 20% solution of1,2, 4-trichlorobenzene/d₆ -benzene and run on a Bruker AM 300 WBspectrometer using the inverse gate broad band decoupling method. Theexperimental conditions were: transmitter frequency 75.47 MHz; decouplerfrequency 300.3 MHz; pulse repetition time 12 seconds; acquisition time1.38 seconds; pulse angle 90° (11.5 microseconds pulse width); memorysize 74K points; spectral window, 12195 Hz. Seven thousand transientswere accumulated, and the probe temperature was set at 133° C. The NMRspectrum for the polymer produced and recrystallized from xylene onetime is shown in FIG. 2. The calculated and observed values for thespectra are shown in Table 3 with Example 1 representing the data forthe sample recrystallized once from xylene and Example 1-A representingthe data for the sample recrystallized three times from xylene. Thecalculated values were derived using the Bernoullian probabilityequations as disclosed in Inoue Y., et al, Polymer, Vol. 25, page 1640(1984) and as known in the art.

The results show that in the sample recrystallized once from xylene thepercentage of racemic dyads (r) is 95%. For the sample recrystallizedthree times from xylene the percentage of r dyads is 98% indicating apolymer that consists of 2% or less of the meso (m) dyad. Further, theNMR spectrum shows that the meso dyads occur predominately in pairs,i.e., mm triads, as opposed to the previously known single m dyadstructure in the chain. Thus, the catalysts of the present inventionproduce a polymer product with a novel microstructure from thatpreviously known.

EXAMPLE 2

The procedures of Example 1 were repeated except that 500 ml of toluenewas used as a co-solvent in the polymerization reaction. Further, onegram of MAO was used in the polymerization, and the reaction temperaturewas 50° C. Fifteen grams of oil were obtained along with the polymerproduct. The polymer was analyzed in accordance with the proceduresgiven above and the results are shown in Table 1.

EXAMPLE 3

The procedures of Example 2 were repeated except that hafnium was usedas the transition metal in the catalyst. The other reaction conditionswere as shown in Table 1, and the analyzed properties of the resultingpolymer are also shown in Table 1.

FIGS. 4 and 5 show the IR spectra for the polymer produced in Examples 7and 8 respectively. The characteristic bands at 977 and 962 cm⁻¹ forsyndiotactic polypropylene are readily visible. The presence of thesebands reaffirm the syndiotactic structure of the polymer. Thecorresponding bands for isotactic polypropylene are 995 and 974respectively.

EXAMPLES 4 THROUGH 8

The procedures of Example 1 were repeated except for the differingreaction conditions as shown in Table 1. In addition, Example 4 usedchromotography as the purification procedure and Example 5 utilized nopurification procedure. The results of the polymerization and theanalysis of the polymer are shown in Table 1.

FIGS. 3 and 4 show the IR spectra for the polymers produced in Examples7 and 8 respectively with the polymer recrystallized three times.

EXAMPLES 9-16

The procedures of Example 1 were repeated except for the changes in theamounts of catalyst and cocatalyst as indicated in Table 1. Further, thecatalysts in Examples 9-13 and 15 were purified using both extractionwith pentane and fractional recrystallization. Example 14 usedextraction with pentane and chromotography as the purificationprocedures. Example 16 did not use any purification procedure.

EXAMPLE 17

The procedures of Example 1 were repeated except that hafnium was usedas the transition metal for the catalyst. The other reaction conditionswere as shown in Table 1. The catalyst was purified using extractionwith pentane and fractional recrystallization. The results of thepolymerization are shown in Table 1.

EXAMPLES 18 AND 19

A hafnium metallocene catalyst was synthesized using Method B asdescribed above and using the 95% pure HfC1₄ that contained about 4%ZrC1₄. The polymerization was carried out using the polymerizationprocedures of Example 1 under the conditions shown in Table 2. Thepolymers were analyzed in accordance with the procedures set forth inExample 1 and the results are shown in Table 2.

EXAMPLES 20-31

A zirconium metallocene catalyst was prepared using the synthesisprocedures of Method B, and the polymerization of propylene was carriedout under the conditions shown for each Example in Table 2. The polymerproducts were analyzed in accordance with the procedures of Example 1and the results are given in Table 2. It should be noted that forExamples 20-22, the syndiotactic index, S.I., was determined for thexylene insoluble fraction. The syndiotactic index for these fractionswere nearly 100%. The observed (obsd.) NMR spectra data for Examples 20and 22 are shown in Table 4. The data given for Examples 20 and 22 wascollected from the polymers produced in Examples 20 and 22 respectivelyand recrystallized once from xylene. Example 22-A is the polymer ofExample 22 that is recrystallized three times from xylene.

EXAMPLES 32-33

A hafnium metallocene catalyst was prepared using the synthesisprocedures of Method B. The catalyst for Example 32 was prepared usingthe 99% pure HfC1₄ while the catalyst in Example 33 was prepared fromthe 95% pure HfC1₄ that contained about 4% ZrC1₄. The polymerization wascarried out in accordance with the procedures of Example 1 under theconditions shown for Examples 32 and 33 in Table 2. The results of theanalysis of the polymer produced in these Examples are also shown inTable 2. The NMR data for Example 33 is shown in Table 4 with the sampleas recrystallized once from xylene (Ex. 33) and three times from xylene(Ex. 33A).

The data shown in Tables 1-4 and in FIGS. 2 and 3 show that thecatalysts of the present invention produce a predominantly syndiotacticpolymer that has high crystallinity and a novel microstructure.Particularly, the NMR data shown in Tables 3 and 4 establish that thexylene insoluble fraction consists of a very high percentage ofsyndiotactic polymer with very little, if any, isotactic polymer beingproduced. Further, the syndiotactic polymer contains a high percentageof "r" groups and "rrrr" pentads indicating that there is only a smallpercentage of deviations from the "...rrrr..." structure in the polymerchain. The deviations that do exist are predominantly of the "mm" type.Indeed, the results for Ex. 1-A in Table 3 show that the only deviationin the chain is of the "mm" type. The other NMR samples show thepredominance of the "mm " deviation over the "m" deviation. Thus, anovel microstructure for syndiotactic polypropylene has been discovered.

The data in Tables 1 and 2 shows the high crystallinity of the polymerproduct. The relatively high melting points, TM1 and TM2, and therelatively high heats of crystallization, -Hc, indicate that thepolymers are highly crystalline. The data further indicates a corelationbetween the polymerization reaction temperature, T, and the meltingpoints, molecular weights and the heats of crystallization of thepolymer. As the reaction temperature increases, all three of theseproperties decrease. There also seems to be a range of temperaturewithin which the yield of polymer is maximized. This temperature rangewill vary with the type of catalyst used but is typically 50-70° C. Theconcentration of methylalumoxane (MAO) also appears to affect thepolymer yield. The data indicates that to a point, the greater theconcentration of MAO, the higher the yield of polymer. The concentrationof MAO also seems to have some effect on the amount of atactic polymerproduced. MAO appears to act like a scavenger for impurities and tendsto reduce the amount of atactic polymer produced.

The data further indicates a difference between the zirconium catalystsand the hafnium catalysts of the present invention. The polymersproduced with the hafnium catalysts tend to be less crystalline and havelower melting points than the polymers produced with the zirconiumcatalysts. The data in Table 4 also shows that the hafnium catalystproduces a higher percentage of isotactic blocks in the polymer chain asreflected by the presents of the isotactic pentad mmmm.

Examples 18, 19 and 33 show the ability to achieve a broader molecularweight distribution, MWD═Mw/Mn, by use of a mixture of two or more ofthe catalysts described by the present invention. The catalysts in theseExamples were prepared using HfC1₄ that contained about 4% ZrC1₄. TheMWD of the polymer in these Examples is significantly higher than theMWD of the polymer produced by an essentially pure hafnium catalyst--see Example 32. Thus, a mixture of two different catalysts can be usedto produce a polymer with a broad MWD.

It should be further understood that the syndiospecific catalysts of thepresent invention are not limited to the specific structures recited inthe Examples, but rather, include catalysts described by the generalformula given herein in which one Cp ring is substituted in asubstantially different manner so as to be sterically different. In theExamples above, the rings included an unsubstituted Cp ring and a Cpring substituted to form a fluorenyl radical, but similar results areobtainable through the use of other ligands consisting of bridged Cprings in which one of the Cp rings is substituted in a substantiallydifferent manner from the other Cp ring, e.g., an indenyl radical and aCp ring, a tetramethyl substituted Cp ring and a Cp ring, a dialkylsubstituted Cp ring and a monoalkyl substituted ring, etc.

From the detailed description of the invention just given, it isapparent that the invention provides a catalyst and a process forpreparing syndiotactic polyolefins. Having described but a fewembodiments, it will be apparent to one skilled in the art that variousmodifications and adaptations may be made to the catalysts and processesas described without departing from the scope of the present invention.

                                      TABLE 1                                     __________________________________________________________________________    Method A                                                                               Catalyst                                                                           MAO     Yield                                                   Hc                                                                            Example                                                                            Metal                                                                             (mg) (cc)                                                                              T ° C.                                                                     (g) Tm 1° C.                                                                    Tm 2° C.                                                                    J/g                                                                              Mp/1000                                                                            Mw/Mn                                                                              S.I. %                       __________________________________________________________________________    1    Zr  10.0 10.0                                                                              20  14  145  150  43 118  2.5  62                           2    Zr  10.3 1 g 50  26  129  137  45 57   1.9  68                           3    Hf  10.3 1 g 50  12       104  17 1222      46                           4    Zr  5.0  10.0                                                                              50  130 132  138  37 61        87                           5    Zr  5.1  10.0                                                                              50  83  131  138  38 62        84                           6    Zr  5.0  0.3 g                                                                             70  22  115  127  34 71        83                           7    Zr  5.1  5.0 50  68  131  140  37 60        38                           8    Zr  5.1  10.0                                                                              50  110 132  140  38 60        42                           9    Zr  5.1  1.0 50  14  114  126  21 58        24                           10   Zr  5.0  2.5 50  34  111  122  14 60        23                           11   Zr  5.1  5.0 50  68  119  130  21 60        38                           12   Zr  5.0  10.0                                                                              50  78  128  137  32 64        65                           13   Zr  5.0  1.0 50  83  121  132  22 59        42                           14   Zr  2.6  10.0                                                                              50  85  134  140  40 62        89                           15   Zr  5.1  10.0                                                                              50  110 125  134  29 60        42                           16   Zr  5.1  10.0                                                                              50  115 131  138  38 62        84                           17   Hf  10.3 1 g 80  55  89   108     223       52                           __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    METHOD B                                                                               Catalyst                                                                           MAO     Yield                                                   Hc                                                                            Example                                                                            Metal                                                                             (mg) (cc)                                                                              T ° C.                                                                     (g) Tm 1° C.                                                                    Tm 2° C.                                                                    J/g                                                                              Mw/1000                                                                            Mw/Mn                                                                              S.I. %                       __________________________________________________________________________    18   Hf  10.0 10  50  58  116  125  24 644  5.4                               19   Hf  5.0  10  50  60  117  124  24 774  4.8                               20   Zr  0.6  10  50  162 134  140  40 69   1.8  95                           21   Zr  1.5  10  29  49  142  146  45 106  1.9  95                           22   Zr  0.6  10  70  119      134  39 54   2.0  95                           23   Zr  0.2  10  50  27  135  140  39 69   1.9                               24   Zr  0.6  10  50  162 134  140  40 69   1.8                               25   Zr  0.6  10  25  26       145  44 133  1.9                               26   Zr  0.6  10  70  119      134  39 54   2.0                               27   Zr  1.5  10  29  49  142  146  45 106  1.9                               28   Zr  2.5  10  50  141 135  141  40 70   1.9                               29   Zr  5.0  10  28  152 128  137  43 88   2.1                               30   Zr  0.5  10  60  185 128  137  37 52   1.8                               31   Zr  0.5  5   70  158 120  134  36 55   2.4                               32   Hf  2.5  10  70  96  103       19 474  2.6                               33   Hf  10.0 10  50  27  114       26 777  5.3                               __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                               Ex. 1         Ex. 1A                                                            obsd. %  calc. %    obsd. %                                                                              calc. %                                   % r      95       95         98     98                                        ______________________________________                                        mmmm     0.3      0.2        0      0                                         mmmr     0.3      0.6        0      0                                         rmmr     1.5      1.4        1.3    1.0                                       mmrr     2.4      2.9        1.9    2.1                                       rrmr +                                                                        mmrm     1.5      1.6        0      0                                         mrmr     1.6      0.8        0      0                                         rrrr     88.0     89.1       94.7   94.7                                      mrrr     3.9      3.1        2.2    2.1                                       mrrm     0.4      0.4        0      0                                         dw                0.2               0.1                                       ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        EX. 20       EX. 22   EX. 22-A EX. 33 EX. 33-A                                obsd. %      obsd. %  obsd. %  obsd. %                                                                              obsd. %                                 ______________________________________                                        mmmm    0        0.77     0.51   2.34   2.04                                  mmmr    0.23     0.45     0.31   0.73   0.76                                  rmmr    1.67     1.82     1.81   2.72   2.96                                  mmrr    3.58     4.25     4.06   5.72   6.44                                  mrmm +  2.27     3.23     3.57   2.87   3.12                                  rmrr                                                                          mrmr    1.51     2.06     1.70   1.37   1.53                                  rrrr    82.71    77.58    78.12  75.7   74.55                                 mrrr    6.45     7.75     9.02   7.4    8.01                                  mrrm    0.68     0.73     0.93   1.08   0.55                                  ______________________________________                                    

We claim:
 1. A metallocene catalyst for use in preparing syndiotacticpolyolefins, said catalyst described by the formulaR"(CpR_(n))(CpR'_(m))MeQ_(k) wherein each Cp is a cyclopentadienyl orsubstituted cyclopentadienyl ring; each R_(n) is the same or differentand is a hydrocarbyl radical having 1-20 carbon atoms; each R'_(m) isthe same or different and is a hydrocarbyl radical having 1-20 carbonatoms; R" is a structural bridge between the Cp rings impartingstereorigidity to the catalyst; Me is a group 4b, 5b, or 6b metal fromthe Periodic Table of Elements; each Q is a hydrocarbyl radical having1-20 carbon atoms or is a halogen; 0 ≦ k ≦ 3; 0 ≦ n ≦ 4; 1 ≦ m ≦ 4; andwherein R'_(m) is selected such that (CpR'_(m)) is a stericallydifferent ring than (CpR_(n)).
 2. The catalyst of claim 1 wherein R'_(m)is selected such that (CpR'_(m)) forms a fluorenyl or indenyl radical.3. The catalyst of claim 1 wherein Me is titanium, zirconium or hafnium.4. The catalyst of claim 1 wherein R" is selected from the groupconsisting of an alkylene radical having 1-4 carbon atoms, a siliconhydrocarbyl radical, a germanium hydrocarbyl radical, a phosphorushydrocarbyl radical, a nitrogen hydrocarbyl radical, a boron hydrocarbylradical, and an aluminum hydrocarbyl radical.
 5. The catalyst of claim 1wherein R" is a methyl, ethyl, isopropyl, cyclopropyl, dimethylsilyl,methylene or ethylene radical.
 6. The catalyst of claim 1 wherein n is0.
 7. The catalyst of claim 1 wherein R"(CpR_(n))(CpR'_(m)) forms anisopropyl(cyclopentadienyl-1-fluorenyl) radical.
 8. The catalyst ofclaim 1 wherein R_(m) is selected such that (CpR'_(m)) forms afluorenyl, indenyl, tetra-, tri-, or dialkyl substitutedcyclopentadienyl radical and R_(n) is selected such that (CpR_(n)) formsan alkyl substituted or unsubstituted cyclopentadienyl radical.
 9. Thecatalyst of claim 1 further comprising an aluminum compound selectedfrom the group consisting of alumoxanes, alkyl aluminums and mixturesthereof.
 10. The catalyst of claim 9 comprising an isolated complex ofthe metallocene catalyst and the aluminum compound.
 11. A process forpreparing a bridged metallocene catalyst having sterically differentcyclopentadienyl rings comprising:(a) contacting a cyclopentadiene orsubstituted cyclopentadiene with fulvene or a substituted fulvene underreaction conditions sufficient to produce a bridged dicyclopentadiene orsubstituted dicyclopentadiene, and (b) contacting said bridgeddicyclopentadiene or substituted dicyclopentadiene with a metal compoundof the formula MeQ_(k) under reaction conditions sufficient to complexthe bridged dicyclopentadiene or substituted dicyclopentadiene with themetal compound to produce a bridged metallocene, wherein Me is a group4b, 5b or 6b metal from the Periodic Table of Elements, each Q is ahydrocarbyl radical having 1-20 carbon atoms or is a halogen and 0 ≦ k ≦4.
 12. The process of claim 11 wherein the contacting of step (b) isperformed in a chlorinated solvent.